Section 1
Four basic steps are involved in getting an MR image—
- Placing the patient in the magnet
- Sending Radiofrequency (RF) pulse by coil
- Receiving signals from the patient by coil
- Transformation of signals into image by complex processing in the computers.
Now let us understand these steps at molecular level. Present MR imaging is based on proton imaging. Proton is a positively charged particle in the nucleus of every atom. Since hydrogen ion (H+) has only one particle, i.e. proton, it is equivalent to a proton. Most of the signal on clinical MR images comes from water molecules that are mostly composed of hydrogen.
How do protons help in MR imaging?
Protons are positively charged and have rotatory movement called spin. Any moving charge generates current. Every current has a small magnetic field around it. So every spinning proton has a small magnetic field around it, also called magnetic dipole moment.
Normally the protons in human body (outside the magnetic field) move randomly in any direction. When external magnetic field is applied, i.e. patient is placed in the magnet, these randomly moving protons align (i.e. their magnetic moment align) and spin in the direction of external magnetic field. Some of them align parallel and others anti-parallel to the external magnetic field. When a proton aligns along external magnetic field, not only it rotates around itself (called spin) but also its axis of rotation moves forming a ‘cone’.2
Fig. 1.1: Spin versus precession.
Spin is rotation of a proton around its own axis while precession is rotation of the axis itself under the influence of external magnetic field such that it forms a ‘cone’
This movement of the axis of rotation of a proton is called as precession (Fig. 1.1).
The number of precessions of a proton per second is called precession frequency. It is measured in Hertz. Precession frequency is directly proportional to strength of external magnetic field. Stronger the external magnetic field, higher is the precession frequency. This relationship is expressed by Larmors equation—
Wo | = γ Bo |
Where wo = precession frequency in Hz
Bo | = Strength of external magnetic field in Tesla |
γ | = Gyromagnetic ratio, which is specific to particular nucleus |
Precession frequency of the hydrogen proton at 1, 1.5 and 3 Tesla is roughly 42, 64 and 128 MHz respectively.
Longitudinal Magnetization
Let us go one step further and understand what happens when protons align under the influence of external magnetic field. For the orientation in space consider X, Y, and Z axes system. External magnetic field is directed along the Z-axis. Conventionally, the Z-axis is the long axis of the patient as well as bore of the magnet. Protons align parallel and antiparallel to external magnetic field, i.e. along positive and negative sides of the Z-axis.3
Forces of protons on negative and positive sides cancel each other. However, there are always more protons spinning on the positive side or parallel to Z-axis than negative side. So after canceling each others forces there are a few protons on positive side that retain their forces. Forces of these protons add up together to form a magnetic vector along the Z-axis. This is called as longitudinal magnetization (Fig. 1.2).
Longitudinal magnetization thus formed along the external magnetic field can not be measured directly. For the measurement it has to be transverse.
Transverse Magnetization
As discussed in the previous paragraph when patient is placed in the magnet, longitudinal magnetization is formed along the Z-axis. The next step is to send radiofrequency (RF) pulse. The precessing protons pick up some energy from the radiofrequency pulse. Some of these protons go to higher energy level and start precessing antiparallel (along negative side of the Z-axis). The imbalance results in tilting of the magnetization into the transverse (X-Y) plane. This is called as transverse magnetization (Fig. 1.3). In short, RF pulse causes titling of the magnetization into transverse plane.
The precession frequency of protons should be same as RF pulse frequency for the exchange of energy to occur between protons and RF pulse. When RF pulse and protons have the same frequency protons can pick up some energy from the RF pulse.4
Fig. 1.3: Transverse magnetization.
Magnetization vector is flipped in transverse plane by the 90 degree RF pulse
This phenomenon is called as “resonance”- the R of MRI.
RF pulse not only causes protons to go to higher energy level but also makes them precess in step, in phase or synchronously.
MR Signal
Transverse magnetization vector has a precession frequency. It constantly rotates at Larmor frequency in the transverse plane and induces electric current while doing so. The receiver RF coil receives this current as MR signal (Fig. 1.4). The strength of the signal is proportional to the magnitude of the transverse magnetization. MR signals are transformed into MR image by computers using mathematical methods such as Fourier Transformation.
Revision:
Basic four steps of MR imaging include:
- Patient is placed in the magnet—All randomly moving protons in patent's body align and precess along the external magnetic field. Longitudinal magnetization is formed long the Z-axis.
- RF pulse is sent—Precessing protons pick up energy from RF pulse to go to higher energy level and precess in phase with each other. This results in reduction in longitudinal magnetization and formation of transverse magnetization in X-Y plane.5Fig. 1.4: MR signal.The TM vector starts reducing in its magnitude immediately after its formation because of dephasing of protons. The LM starts gradually increasing in its magnitude. The resultant net magnetization vector (NMV) formed by addition of these two (LM and TM vectors) gradually moves from transverse X-Y plane into vertical Z-axis. As long as the NMV is in the transverse plane it produces current in the receiver coil. This current is received by the coil as MR signal
- MR signal is received—The transverse magnetization vector precesses in transverse plane and generates current. This current is received as signal by the RF coil.
- Image formation—MR signal received by the coil is transformed into image by complex mathematical process such as Fourier Transformation by computers.
Localization of the Signal
Three more magnetic fields are superimposed on the main magnetic field along X, Y, and Z axes to localize from where in the body signals are coming. These magnetic fields have different strength in varying location hence these fields are called “gradient fields” or simply “gradients”. The gradient fields are produced by coils called as gradient coils.
The three gradients are—
- Slice selection gradient
- Phase encoding gradient
Slice Selection Gradient
Slice selection gradient has gradually increasing magnetic field strength from one end to another (Fig. 1.5). It determines the slice position. Slice thickness is determined by the bandwidth of RF pulse. Bandwidth is the range of frequencies. Wider the bandwidth thicker is the slice.
Phase Encoding and Frequency Encoding Gradients
These gradients are used to localize the point in a slice from where signal is coming. They are applied perpendicular to each other and perpendicular to the slice selection gradient (Fig. 1.6).
Typically, for transverse or axial sections following are axes and gradients applied even though X and Y axes can be varied.
- Z-axis—Slice selection gradient
- Y-axis—Frequency encoding gradient
- X-axis—Phase encoding gradient.
In a usual sequence, slice selection gradient is turned on at the time of RF pulse. Phase encoding gradient is turned on for a short time after slice selection gradient. Frequency encoding or readout gradient is on in the end at the time of signal reception.
Information from all three axes is sent to computers to get the particular point in that slice from which the signal is coming.
Why Proton only?
Other substances can also be utilized for MR imaging. The requirements are that their nuclei should have spin and should have odd number of protons within them. Hence theoretically 13C, 19F, 23Na, 31P can be used for MR imaging.
Hydrogen atom has only one proton. Hence H+ ion is equivalent to a proton. Hydrogen ions are present in abundance in body water. H+ gives best and most intense signal among all nuclei.